Process of making a semi-conductive ceramic body



1958 5. J. SHEHEEN ET AL 2,361,014

PROCESS OF MAKING A SEMI-CONDUCTIVE; CERAMIC BODY Filed Au 14, 1956' ml M 5/ IN V EN TORS 5H4) f JHEHEf/V fDW/MD Z. MOO/V57 fawn/ m MAKING A SEMI-CONDUCTIVE CERAMIC BODY Shay J. Sheheen, Ithaca, and'Edward L. Mooney, Norwich, N. Y., assignors to General Laboratory AssocYlatelf, .Inc., Norwich, N. Y., a corporation of New v Application August 14, 1956, Serial No. 603,971

7 Claims. (Cl. 117-219) PROCESS OF The present invention relates to spark-producing devices or igniters, semi-conductive ceramic elements for use 'therein and the process of making them. Such devices as herein disclosed are particularly useful in igniting fuel and air mixtures in internal combustion engines and particularly in jet engines of the type used in aircraft. Various phases of the invention have, however, much wider utility, for example, in spark-producing devices in general. a

' -Spark plugs or igniters of the common type used on internal combustion engines have a pair of electrodes separated by an air gap in which the spark is produced. Such igniters commonly include a member of insulating material which is disposed between the electrodes and has a surface adjacent to the air gap. When in use, such igniters are at times subject to a condition commonly termed fouling, characterized by the formation of a deposit, usually carbonaceous in character, on the insulator'surface. The carbon deposit provides a more or less electrically conductive path in parallel with the spark gap and weakens the spark, or may, if sufliciently conductive, prevent the formation of a spark.

It has also been proposed to provide an igniter of the so-called surface gap type. In such igniters, the electrodes are separated by a semi-conductive member, rather than a member of insulating material. Portions of the surface of this member, including particularly the metalto-semi-conductor contact area, are believed to be heated by the electric current when a discharge is initiated between the electrodes, and are thereupon believed to give off electrons which aid in breaking down the gap, so that the main discharge follows as a spark between the electrodes and just above the conductive surface. Surface gap igniters have the advantage of a lower voltage requirement for a given spark energy. Furthermore, the heating of the conductive surface during the discharge tends to burn off any fouling deposit. However, in igniters of this type constructed in accordance'with the teachings of the prior art, the energy dissipated in the spark may vary considerably under fouling conditions, as the energy dissipated in the fouling deposit varies with its thickness and composition.

In internal combustion engines ignition takes place following a compression stroke in a chamber having a high temperature, a high pressure, and a reasonably uniform mixture of fuel and air. Under such conditions, successful ignition of the mixtures is comparatively easy. As long as some kind of spark occurs, the fuel mixture will ignite.

In jet engines, however, the conditions are quite different and the problem of producing satisfactory ignition of the fuel is considerably more complicated. The pressure may vary widely, for example, between about 3 inches of mercury (absolute) and 250 pounds per square inch (gage) pressure. The temperatures at the point of ignition also may vary widely, for example, from about -,65 F. to about 2000 F. There is commonly a great amount of turbulence in the neighborhood of the igniter,

Patented Nov. 18, 195.8

,2 and there isa wide variation in the fuel-air ratio of the mixture presented for ignition. e e I Because-of these considerations, it is desirable in such engines to provide ignition apparatus which will produce sparks of uniformly high energy.

) In addition to the low temperature requirements during I starting conditions, an igniter for a jet engine must be able to withstand very high temperatures, for example, of, the order oflSOOY-ZOOO F., during operation ofthe engine. r The present invention relates to igniters and similar spark producing devices of the surface gap type. One of the principal features of the invention is the use of a semi-conductive material to form the active surface of the,

surface gap. e a

' The, term semi-conductivematerials is a well recognized generic term for a large group of materials which are neither good electrical conductors nor good electrical insulators. These materials commonly have a negative, temperature coefficient of electrical resistance. In otherv words, their resistance decreases as their temperature in: creases. This characteristic is just the opposite of the corresponding characteristic of electrical conductors,the resistance of which commonly increases as their temperature increases. I, v

In a surface gap type of igniter, the energy dissipated in the spark, as contrasted to the energy dissipated in the conducting surface, is believed to be determined to a great extent by the emission of electrons from the surface.

The greater the quantity of electrons emitted, the quicker the gap throughthe air breaks down, and the greater the; quantity ofenergy dissipated in the gap rather than in the conducting surface.

While it has .been proposed heretofore to produce the ceramic element of such an igniter, either formed in its entirety ofsemi-conductive material or having a layer thereof on the side facing and in contact with the electrodes of the spark-producing device or igniter, it has been found desirable in accordance withthe present invention to employ a ceramic element made of more or lesscon:

ventional ceramic material which is electrically nonconductive and which is calculated to stand up under the physical strains to which it is exposed, both mechanical and due to temperature, with the surface portion only of this ceramic element made semi-conductive by impregnation. For example, it has been found that if a body which is largely or substantially wholly alumina is surface-impregnated with a suitable metallic oxide which,

will provide semi-conductive characteristics to the surface portion, the objects desired for a device of this kind will be attained. These objects embrace the ability to withstand any and all physical shocks and vibration to which.

devices of this kind may be exposed in use as well as the ability to withstand the temperatures to which such devices are exposed in modern jet engines, i. e., temperatures in the order of 1800-2000 F. as aforesaid. It is further the purpose of the present invention to provide a' ceramic element and a sparking device formed therewith, wherein the surface portion of the ceramic element is semi-conductive to the end that it has an'electric resistance of the order of about 5,00030,000 ohms between points which are 0.04 inch apart.

It is a further object of the present invention, which is attained by following the process thereof using the materials hereinafter set out in detail to provide a each elfective on a layer of metallic oxide material, such as or' including copper oxide, which shall produce an impregnated semi-conductive surface portion on a nonconductive ceramic body as hereinabove generally referred to;

Other objects and, advantages of the present invention willbecome apparent from a consideration of the followingspecification and claims, all when taken in connection with the accompanying'drawings', .in which:

Fig: 1 is an electrical wiring diagram of an ignition system suitableyfor use with igniters constructed in accordance with the invention;

Fig. 2is' a longitudinal, cross-sectional view of an. igniter embodying the invention; and

Fig; 3'-isacross-sectionalview of'a ceramic element whichmay be used in the igniter of Fig. 2.

The ignition system illustrated in Fig. 1 includes mechanism for producing a uniform charge as disclosed and claimed in the U. S. Patent to John V. McNulty, No. 2,716,720; issued August 30, 1955, for Engine Ignition Apparatus and Procedure. Systems of this type have the characteristic-that charges of electricity for theformation'of sparks" are delivered to the igniters in carefully measured and regulated quantities of energy, so that uniform sparks may be produced at the igniter, regardless of variations which mayoccur in the energy supply system; for example, in the potential of the source of electrical energy.

Referring to the drawing, there is shown a battery 1 which supplies electrical energy to a motor 2, drivingly' connected to a multiple lobe cam 3 and a single lobe cam 4; The multiple lobe cam 3 operates a switch 5*connected in'series between the battery 1 and the primary section 6 of the winding of anauto-transformer 7'. The secondary section 8 of the winding'of the autotransformer 7 is connected in series with a circuit which includes a rectifier or other asymmetric element 9, and a condenser 10; Two branch circuits are connected to the condenser 10, each branch including a trigger switch 11 operated bythe single lobe cam 4, an igniter 12, and a-resistance13 in parallel with the plug 12.

The motor 2 has a voltage-speed characteristicsuch that the speed varies directly and linearly as the voltage. Consequently, the time of closure of the switch 5 by the lobe of-the cam 3 varies inversely with the voltage of the source 1. Consequently,regardless of variations in the'voltage of the source, each closure of the switch Sdelivers a fixed predetermined quantity of electricity to the primary section 6 of the winding of the autotransformer 7. These measured quantities are stepped up in potential by the auto-transformer, and pass through the rectifier 9 and are stored on the condenser 10. The condenser 10 is discharged each time that the single lobe cam 4 actuates one of the trigger switches 11.. Since the single lobe cam 4 is driven by the same motor 2' which drives the multiple lobe cam 3, it will be apparent that each discharge of the condenser 10 is preceded by the same number of charging pulses, determined by the number of lobes on one-half the periphery of the cam 3. The condenser 10 discharges through each igniter 12 and the resistance 13 in parallel, respectively. The resistance 13 is a relatively high. resistance, and is effective to discharge the condenser 10 in the event that there is an open circuit at the igniter 12. The purpose of so discharging the condenser 10 is to prevent the accumulation of charges thereon beyond the normal quantity, which might result in the application of ab' normal currents and voltages to the igniter 12.

As shown in Fig. 2, the igniter 12 comprises an outer' shell 14 having at its right-hand end an inwardly projecting flange 14a. The right-hand end of this shell forms the outer or grounded electrode of the igniter. Extending axially within the shell 14 is a central electrode assembly generally indicated at 17 and comprising a body 17a having on its right-hand end (as seen in Fig. 2) an internally threaded recess in which is inserted a bolt 17b having a head which forms the other electrode of the igniter. A ceramic element 21 having a stratum of semiconductive material 23 on its right-hand surface (as seen in Fig. 2) is retained between the body 17a and the head of bolt 17c. A washer 17d is located under the head 17c of the bolt, between that head and the semi-conductive stratum 23. The bolt 17b and the body 17a are preferably formed of, stainless steel. The washer 17d is preferably formed of a nickel manganese alloy known commercially as Nickel-D. This nickel washer provides an electricalcontact between the bolt 17b and the semi-conductive impregnated surface portion 23. The washer is also considerably softer and more resilient than the stainless steel, so that it distributes the compressive stress applied to .the ceramic element 21 when the bolt is tightened in the body 17a, The ceramic element 21 is thereby stressed only in compression which it is capable of withstanding readily. Shear and tension stresses which might crack the ceramic material are thereby avoided. After the ceramic element 21 and washer 1721 are assembled on the end of body 17a by means of the bolt 17b, the bolt and 'the body may be welded together electrically and a screw slot (not shown) which was formed on the head 170 to facilitate assembly, is ground or machined off.

After the central. electrode assembly 17 is complete, it is mounted in the shell 14. A washer 22, of material similar to the washer 17d, is located between the righthand periphery of the ceramic element 21 and the flange 14a of the shell. A plurality of longitudinally sec-. tional ceramic spacers 18 (shown on the drawing as a single element) are then inserted behind the ceramic element 21;

A terminal 19 is attached to the left-hand end of the body 17a, as by welding. A copper washer 20b encircles the terminal 19adjacent to the body 17a. A bushing 20 is receivedina recess formed in the left-hand end of the spacers 18 and abuts against washer 20b. A collar 20a of suitable insulating or nonconductive material encircles the bushing 20 and abuts against the spacers 18. The

collar 20a has an inwardly directed flange on its left-' hand end to engage the bushing 20 and hold it in. place.

The shell'.14 is provided with an internal thread on its left-hand end to receive an externally threaded coupling sleeve 15 which abuts against the collar 20:: and holds all the parts firmly in position. A gasket 16 of insulating material is located between the right-hand end 15a (as seen in Fig. 2) of sleeve 15 and a shoulder formed in the shell 14.

When. the sleeve 15 is tightened into the shell 14, it forces the collar 20a against the spacers 18, which are in turn forced against ceramic element 21. The cermic ele-. ment 21 is thereby held in tight contact with the washer 22; The function of washer 22 is' similar to that of washer 17d. That is, it distributesthe compressive stress applied to the ceramic element 21 and prevents. any tension or shear stress on that element. It also provides good electrical contact between the semi-conductive stratum or impregenated surface p0rtion23 and the outer electrode shell 14.

There is shown in Fig. 3 a ceramic element 21 having its. upper or end surface portion 23 impregnated with a semi-conductive material, so as to provide some surface conductivity, which will be equivalent to a body having an electrical resistance of about 5,00030,000 ohms between points 0.04 inch apart. In this. connection, while a resistance range is given between points a stated distance apart, it will be understood that the distance 0.04 inch is not necessarily the spacing between the elec-. trodes 17c and 14a. The resistance between such electrodes will, of course, be greater if the distance is. greater in accordance with the well-known rule, so that the actual resistance between electrodes along the surface of the semi-conductor may be made any desirable amount by suitably controlling the radial spacing between the electrodes and taking into account the actual conductivity of the surface portion 23 of the ceramic element, which is semi-conductive as aforesaid.

The body portion of the ceramic element may be of any suitable ceramic material, having the necessary mechanical strength and resistance to damage by heat. It has been found, however, that a body having at least about 90% alumina is desirable for this purpose. In addition, there may be about 5-7% calcium oxide, about /2 to about l /2% strontium oxide and about 35% silica. Specifically, a preferred composition for this body is alumina 90%; alkaline earth oxides 7%, including, for example, specifically, 6% CaO and 1% SrO; and the balance (4%), silica. This material may be fabricated by conventional ceramic methods, including mixing, drying and firing. Due to the fact that a selected part of the surface portion of this material is to be impregnated with other materials as hereinafter set out, it is 'desirable that the body, the composition of which has been given in detail as a preferred example, shall be made with but a small amount of melting of the ceramic material, so as to have a selectively small amount of glassy matrix bonding together the crystalline particles of the several ceramic oxides used. Such a body may be made, for example, by firing the preferred composition aforesaid for about one hour at about 2850 F. The product of this stage of the process may have a bulk density of about 3.45 (gm./cc.). This product may be ground to any exact size required in cases where such size is critical.

The body portion of the ceramic element and its composition and/or process of formation is susceptible of quite wide variation and is not per se a part of the present invention except when it is suitably combined with the impregnation steps of the process and/or the product thereof. This impregnation is preferably accomplished in two steps, in the first of which a selected portion of the surface of the body portion of the element as aforesaid is impregnated with a material, the essential active ingredient of which is an oxide of copper, such as. cuprous oxide (Cu O).

As a practical matter, it has been found convenient in effecting this first stage'impregnation to spread upon the surface of the area to be impregnated a paste-like material made up as hereinafter set forth of a copper oxide such as Cu O with the balance CaO. A preferred composition of these materials, as to the solid material portion thereof, is Cu O about 80% to about 90% and CaO about 20% to about 10%. These proportions are not believed to be critical. It is believed that the presence of some CaO may promote the conductivity of the coating or impregnated surface. It is believed that some fusion of the Cu O, CaO and A1 0 may occur at about 2100 F., thereby forming (on cooling) a footing having a. relatively high resistance but which serves to support the second coating hereinafter set forth.

The making up of a paste-like form of material for the coating operation again is not critical, as merely enough water should be admixed with the dry material to make a paste of the desired consistency. In practice, it

has been found that about 10 grams of dry material admixed with about 10 grams of water will make up a paste of a desired consistency for use. It will be understood, of course, that the water is evaporated completely during the subsequent firing and further that any alkaline earth hydroxide formed will be reconverted to the corresponding oxide during and as a result of the firing, so that it is the composition of the original dry materials which is pertinent as to the resulting product.

Subsequent to the application of a thin layer of the paste-like material, which may be, for example, about one mm. in thickness (assuming a paste composition use a fire which is preferably on the oxidizing side off made up in accordance with the proportions hereinabove given), the next operation is that of firing the coated body. This is preferably effected at a temperature in the".

The temperature limits chosen are selected so as to sufficiently high to facilitate adequate penetration or in:lpregnation of the copper oxide into the more or less porous ceramic body This accounts for the selection of the lower temperature limit as above given, as tempera= tures substantially below this low limit will not enable the copper oxide to migrate into the interstices of theceramic body to a sufficient or desired extent. In view of this manner of determining the lower temperature limit,this limit is not narrowly critical.

The upper temperature limit (2600 F.) is selected on the basis of accomplishing a sufiicient amount of penetration of the copper oxide into the porous ceramic body without, at the same time, fusing enough of the material present at oradjacent to the surface of the ceramic body so as to make that surface relatively impervious to further penetration of an impregnating material during the second treatment as hereinafter set forth. In general the purpose of the first fire inmaturing the first coating is to, provide a good base or footing onto which a second coating, fired by a second fire as hereinafter set forth, will be put. A further reason for choosing the upper tempera ture limited at about 2600 F. as aforesaid is that still higher temperatures might tend to distort the ceramic body. It will be understood that once a desired surface of such a body is coated or impregnated with a semi-con-- ductive material, that surface may not be ground down to correct a distorted shape without at the same time adversely affecting the desired semi-conductive character of the remaining surface.

- It has been found that the electrical resistance of surfaces impregnated with either pure CuO or with pure Cu O are both relatively high (that with Cu 0 further being higher than that with CuO); whereas those surfaces impregnated with a mixture of C and Cu O is relatively lower. analogous to the effect on the melting point of mixtures of materials having a low melting eutectic. A relatively low resistance semi-conductive material is desired in accordance with the present invention, so that it is desired that the surface impregnation be by a mixture of CuO and Cu O.

It is' also known from published data that the melting point of Cu O is about 2255 F.; while CuO decomposes at about 1879 F., probably yielding Cu O and oxygen.

Thus at a relatively high temperature, the only copper oxide material which is stable is Cu O. In view of the fact that a mixture of these two materials is desired, and

as it would be practically impossible to stop a reducing action at some desired intermediate point, it has been found impractical to start with CuO and then use a re-- ducing action during the firing. What is done according to the present invention is to start with Cu O and then neutral, and in any event is non-reducing. For this reason, therefore,'the atmosphere during the first firing operation is non-reducing and is preferably oxidizing. Due to the fact, however, that in most instances at least,

the present process contemplates not one, but two, firing atmosphere and as a reducing atmosphere is practically undesired, due to the fact that it is not desired. to pro;-

This last is somewhat surprising and is probably duce any metallic copper in the coating, the preferred condition to be established and maintained during the first firing isan oxidizing atmosphere.-

' When prepared in this manner, the impregnated surface will be semi-conductive and will have a conductivity of the order such that the electrical resistance between two points 0.04 inch apart is from about 1 to about 10 megohms. While this degree of conductivity may beadequate for some purposes, a much higher degree of conductivity is usually found desirable for spark-producing devices, which is a principal use for the product of the present invention. For this reason, therefore it is usually desirable, if not absolutely necessary, that a further treatment be given the ceramic elements so as to render the semiconductive surface thereof more conductive than it is after the first treatment as aforesaid.

In accordance with the present invention, a second treatment is carried on, preferably in a manner similar to that just described for the first treatment, with the exception, however, that the essential active ingredients of the dry oxide materials used in the second coating, which is fired onto theselected portion of the surface, is somewhat different than in the first treatment and the firing temperature is in a lower range and, further that theatmosphere in which this second firing takes place must be oxidizing in character. In accordance with the present invention, the essential active ingredients for the second coating are cuprous oxide (Cu O) and an alkaline earth metal chromate, such as calcium chromate (CaCrO The proportions for the active materials as aforesaid in the second coating according to the present invention are copper oxide (Cu O) from about 80% to about 90%, and alkaline earth metal chromate (CaCrO from about 20% to about 10%. These proportions and possible variations thereof will be illustrated further in examples which follow.

Here again the proportions of the ingredients of the solid material for the second coat are not believed to be critical, but may be varied to a substantial degree. The purpose of the several ingredients is believed to be substantially the same as to the copper oxide ingredient as in the first coat, i. e., to provide an impregnating material which is itself a semi-conductor and which will impart semi-conductive characteristics to the treated surface portion of the ceramic body. The alkaline earth chromate is believed to act as an oxidizing agent and possibly also as an oxygen carrier during the second firing as further described hereinafter. Again it is preferred that the material be applied in paste-like form, this, however, being merely a matter of convenience as again the liquid used to make the paste (usually water) is completely evaporated during the subsequent firing, and further any hydrolysis products will be reconstituted as oxides incident to the firing. In this connection, it is noted that the alkaline earth metal chromate (CaCrO may be considered as two oxides, CaO and CrO this being a common way of considering ceramic materials, irrespective of the actual physical state in which they are introduced or to which they are converted by the firing and/or melting operations. Practically, it has been found that if 10 grams of the dry material mixture as aforesaid are mixed with 10 grams of water, a pastelike composition of a practical and workable consistency for use will be produced. This paste-like composition is preferably spread onto the same or a part of the same area which was coated in the first coating and firing step to a thickness of about one mm. prior to the second firing operation per se. In this second firing operation, after first drying out the liquid in the paste coating, the temperature is brought up to the range of about 1900" to about 2300 F. and held there for about /2 hour to about 1 hour for the firing operation per se. Here again it will be understood that the time of firing is usually varied inversely with the temperature, so, for example,

8 at the lower temperature limit, the longer firing time is usually employed, and vice versa.

The atmosphere in which the second firing is conducted must be oxidizing in character, as distinguished from either neutral or reducing. This, in conjunction with the presence of an oxidizing agent (the alkaline earth chromate), establishes positive oxidizing conditions. The purpose of this is believed, in accordance with present theories, to result in the conversion of some of the Cu O initiallypresent to CuO. It is recognized that CuO tends to decompose at the temperatures at which the second firing takes place. However, the presence of positive. oxidizing conditions, possibly coupled with the presence of an active oxidizing agent or oxygen carrier causes the positive conversion of some Cu O to CuO and/or prevents the reverse action (decomposition of CuO) from taking place, so that it is the present theory that the resulting material (i. e., that produced following the cooling after the second fire) contains on the treated surface a mixture of CuO and Cu O. Such a material as aforesaid has an electrical resistance lower than that either of pure ,Cu O or. of pure CuO. It is also reasonably certainthat the presence of positive oxidizing conditions substantially prevents the reduction of oxides of copper to; elemental copper and, therefore, retains the semiconductive character of the coated or impregnated surface, as distinguished from a fully conductive surface which would result if the copper thereon were in elemental, form.

As a result of the second firing operation, there is produced a finished ceramic body, the coated or impregnated surface portion of which is semi-conductive and has an electrical conductivity of the order such that the resistance between two points 0.04 inch apart is about 5,000-30,000 ohms. This treated body may then be assembled into a spark-producing device or igniter as hereinabove described and used in the manner in which such devices are conventionally used in jet engines.

From the point of view of the product of the present process, whether that product be considered the igniter as a whole or whether it be considered merely an impregnated ceramic element, this product is believed to be one which can be made by several different processes. For example, in addition to the process hereinabove described, it is believed that a surface-impregnated ceramic body as hereinafter claimed and which will have the functional characteristics of the body just described, may

also be made, for example, by depositing metal as such on the surface of a ceramic body, for example, by metallizing or electrodeposition; and thereafter some or all of this metal may be converted to an oxide form by some type of oxidizing treatment, usually heating to, high temperature in an oxidizing atmosphere. The present invention does not specifically include such processes for manufacturing such a product, but is intended to include the product, irrespective of the method by which it was manufactured.

The invention is further illustrated by the following specific examples:

Example I This example is given to illustrate the making of a .stable type of a ceramic body having a semi-conductive coating thereon with a resistance within the range given hereinabove, but in a higher portion of this range. This product was made as hereinafter set out by firing in the higher ends of the temperature ranges for each of the two firings. As a result, some copper was believed to be lost by volatilization. A product made in this way, however, has a stable resistance, i. e., one which will not increase its resistance during a normal use.

In accordance with this example, the material forming the first coating applied to the ceramic body was made up of 86% Cu O and 14% CaO. The first firing was conducted under somewhat oxidizing conditions, for about one hour, at about 2600 F. After cooling, :1. second Example II When it is desired to make a low impedance type igniter, i. e., one which can be operated with a low breakdownvoltage, the teachings in this second examplemay be followed. In accordance therewith, the first coating to be applied to the basic ceramic body consisted essentially of 91% Cu O and about 9% CaO. The first firing was carried on for about one hour at about 2300" F. in a slightly oxidizing atmosphere. The second coating applied in this instance consisted essentially of about 91% Cu O and about 9% CaCrO The second firing was carried on in the temperature range of about 1950" F. to 2050 F. for about one hour. The product was found to have the characteristics described in this example.

There is herein shown and described an igniter construction including a coated or impregnated ceramic body having a semi-conductive surface. A process for making such a ceramic body has been described in detail. The theories presently believed to be correct as to what occurs during the several steps of the process, have been set out. It will be understood that irrespective of the actual correctness of the theories or lack thereof, the process steps particularly described will produce the stated results in a reproducible manner. Many equivalents of the process steps and other features herein set out will occur to those skilled in the art, particularly in the light of the foregoing disclosure. We do not wish to be limited, therefore, except by scope of the appended claims, which are to be construed validly as broadly as the state of the prior art permits.

What is claimed is:

1. The process of making a ceramic element usable in a spark-producing device, which comprises the steps of applying to a predetermined surface portion of a body of non-conducting refractory oxide material a first coating, the essential active ingredients of which are cuprous oxide and calcium oxide, firing said first coating on said body in a non-reducing atmosphere for about one-half hour to an hour at a temperature in the range of about 2300 F. to about 2600 F.; thereafter applying onto at least a part of said predetermined surface portion a second coating, the essential active ingredients of which are cuprous oxide and an alkaline earth metal chromate, and firing said second coating onto said body in an oxidizing atmosphere for about one-half hour to an hour at a temperature in the range of about 1900 F. to about 2300 10 F., so as to produce on said selected portion of said el ment an impregnated area having an electrical resistance of about 5,000-30,000 ohms between points 0.04 inch apart.

2. The process of making a ceramic element in accordance with claim 1, in which the time of firing of each of said coatings is varied substantially inversely with the variation in the temperature of firing, within the stated ranges as to both time and temperature respectively.

3. The process of making a ceramic element in accordance with claim 1, in which said first coating is fired in an oxidizing atmosphere.

4. The process of making a ceramic element in accordance with claim 1, in which the active ingredients of the first coating consist essentially of: Cu O About to about CaO About 20% to about 10% 5. The process of making a ceramic element in accordance with claim 1, in which the active ingredients of the second coating consist essentially of:

Cu O About 80% to about 90% CaCrO About 20% to about 10% 6. The process of making a ceramic element in accordance with claim 1, in which the active ingredients of the first coating consist essentially of Cu O, about 86% and CaO, about 14%; and in which the active ingredients of the second coating consist essentially of Cu O, about 86% and CaCrO about 14%; in which the temperature of the first firing is about 2600 F., and the temperature range of the second firing is about 20502350 F., both firings being for about one hour and both being in oxidizing atmospheres.

7. The process of making a ceramic element in accordance with claim 1, in which the active, ingredients of the first coating consist essentially of Cu O, about 91% and C210, about 9%; and in which the active ingredients of the second coating consist essentially of Cu O, about 91% and CaCrO about 9%; in which the temperature of the first firing is about 2300 F., and the temperature range of the second firing is about 19502050 F., both firings being for about one hour and both being in oxidizing atmospheres.

References Cited in the file of this patent UNITED STATES PATENTS 1,546,776 Bullimore July 21, 1925 1,893,935 Emorsleben Jan. 10, 1933 2,578,754 Smits DEC. 18, 1951 2,625,922 Sl1lii5 Jan. 20, 1953 FOREIGN PATENTS 164,511 Australia Aug. 9, 1955 I 

1. THE PROCESS OF MAKING A CERAMIC ELELMENT USUABLE IN A SPARK-PRODUCING DEVICE, WHICH COMPRISES THE STEPS OF APPLYING TO A PREDETERMINED SURFACE PORTION OF A BODY OF NON-CONDUCTING REFRACTORY OXIDE MATERIAL A FIRST COATING, THE ESSENTIAL ACTIVE INGREDIENTS OF WHICH ARE CUPROUS OXIDE AND CALCIUM OXIDE, FIRING SAID FIRST COATING ON SAID BODY IN A NON-REDUCING ATMOSPHERE FOR ABOUT ONE-HALF HOUR TO AN HOUR AT A TEMPERATURE IN THE RANGE OF ABOUT 2300*F. TO ABOUT 2600*F., THEREAFTER APPLYING ONTO AT LEAST A PART OF SAID PREDETERMINED SURFACE PORTION A SECOND COATING, THE ESSENTIAL ACTIVE INGREDIENTS OF WHICH ARE CUPROUS OXIDE AND AN ALKALINE EARTH METAL CHROMATE, AND FIRING SAID SECOND COATING ONTO SAID BODY IN AN OXIDIZING ATMOSPHERE FOR ABOUT ONE-HALF HOUR TO AN HOUR AT A TEMPERATURE IN THE RANGE OF ABOUT 1900*F. TO ABOUT 2300* F., SO AS TO PRODUCE ON SAID SELECTED PORTION OF SAID ELEMENT AN IMPREGNATED AREA HAVING AN ELECTRICAL RESISTANCE OF ABOUT 5,000-30,000 OHMS BETWEEN POINTS 0.04 INCH APART. 